1. Field of the Invention
The invention relates to an energy converter for use In conjunction with heating units for foodstuffs, comprising a first self-inductance which acts as frequency determining element in a parallel resonating circuit and furthermore as the primary winding in a transformer for transmission of the energy.
2. Description of the Related Art
It is known to heat foodstuffs by induction heating of the cooking vessel. For this a source of high frequency energy is connected to a coil which couples the energy to the cooking vessel which contains the food. The energy is given off as lines in the bottom of the vessel which has a certain conductivity and is furthermore ferromagnetic so that the lines of force run partly in the material of the vessel, partly contribute hysteresis losses. The construction may be regarded as a transformer with the oil as the primary winding and the cooking vessel as a secondary winding or energy converter which is loaded near short circuit. By a different lay-out of the component similar circuits may be used in a baking oven where a closed volume envelopes the food and the energy converters while the primary winding remains outside.
When constructing an induction heating unit for food for use in the home it is essential that there is a high efficiency for the conversion of the energy from the mains into high frequency energy since a low efficiency causes losses and hence development of heat in other places than at the cooking vessel where it is required. This development of heat occurs predominantly in coils and semiconductors, and the compact construction of equipment which is required today results in difficulties in removing the developed heat. Several principles for converting the energy may be used, e.g. an oscillator, the output signal of which is amplified in an otherwise linear amplifier or a self-oscillating power oscillator. It has turned out that the latter usually provides the best efficiency.
The distortion in the generated high frequency oscillation must be low because radiation of harmonics may be avoided, which might otherwise cause electromagnetic interference which because of the higher frequency might be annoying. Furthermore higher frequency components would escape more easily as radiation from the energy transferring transformer which are conditions which are regulated by precise health provisions and where higher frequencies are evaluated harder. A requirement for low distortion means in the case of an oscillator that either the oscillating circuit must have low losses, i.e. a high Q value, or else the feed of energy must occur in such a way that only transients which can be filtered by the resonant circuit are generated. Certain energy conversion methods entail a distortion of the current which, however, does not give any problems in practice as regards the output current, unless semiconductor elements are controlled to break the current at a value different from zero which causes a noise radiation rearwards on the mains. In order to convert mains frequency energy to high frequency energy a first step is to use a rectifier circuit which is usually of a type using both half-periods. As the second step active components may participate to feed energy into the resonant circuit in a so-called inverter circuit. Such active components should for reasons of reliability not be loaded too close to their maximum performance.